In recent years, as clean energy which does not affect the environment, an apparatus which uses thermal energy obtained by collecting sunlight is known. As such an apparatus, a sunlight collecting heat receiving system (hereinafter, referred to as a light collecting heat receiving system) has been developed which generates electric power by converting thermal energy into electric energy by collecting sunlight.
In the above-described light collecting heat receiving system, a tower light collecting type is known as one type which collects sunlight.
In the tower light collecting type, a light collecting heat receiver is disposed at a top portion of a tower portion built on a ground, a plurality of heliostats, which is controlled so as to track the sunlight, is disposed around the tower portion, and the sunlight reflected by the heliostats are guided to the light collecting heat receiver, thereby collecting light and heat.
FIG. 15 is a cross-sectional view showing a light collecting heat receiver of the related art when seen from the axial direction of a casing.
As shown in FIG. 15, a light collecting heat receiver 500 of the related art is installed at a top portion of a tower portion (not shown), and includes a bottomed cylindrical casing 502 and a plurality of heat receiving pipes 503. The casing 502 includes an opening portion 501 to which sunlight reflected by the heliostats are incident. The plurality of heat receiving pipes 503 is arranged along the inner wall surface of the casing 502 so that the axial direction thereof is parallel to the center axis of the casing 502, and receives the sunlight incident into the casing 502.
However, in the above-described light collecting heat receiver 500, there is a problem in that a large temperature difference occurs in the circumferential direction (the outer peripheral surface) of each heat receiving pipe 503. Specifically, sunlight H′ reflected from the heliostats are incident from the opening portion 501 into the casing 502, and travel from the opening portion 501 toward the inner periphery of the casing 502. For this reason, the front surface of the heat receiving pipe 503 in the irradiation direction of the sunlight H′ (the surface, which faces the inside in the radial direction of the casing 502, in the outer peripheral surface of the heat receiving pipe 503) becomes a light receiving surface 503a which can directly receive the sunlight H′, and the rear surface in the irradiation direction of the sunlight H′ (the surface, which faces the outside in the radial direction of the casing 502, in the outer peripheral surface of the heat receiving pipe 503) becomes a non-light receiving surface 503b which cannot directly receive the sunlight H′. As a result, since thermal energy may not be efficiently obtained from the sunlight H′ in the non-light receiving surface 503b, there is a problem in that the temperature difference between the light receiving surface 503a and the non-light receiving surface 503b increases.
In this case, thermal stress is generated by the temperature difference between the light receiving surface 503a and the non-light receiving surface 503b. Accordingly, the heat receiving pipe 503 is deformed, which leads to concern that a concentration of stress may be generated in the heat receiving pipe 503 or a connection portion with a header (not shown) integrating the plurality of heat receiving pipes 503. Further, as for the sunlight H′, the amount of solar radiation changes depending on the cycles of day and night, the climate, or the like, the temperature of the heat receiving pipe 503 may change due to the influence of a change in the amount of solar radiation. Then, since the deformation of the heat receiving pipe 503 is repeated by the thermal stress whenever the temperature changes, there is concern that the fatigue life may be reduced.
Therefore, for example, Patent Document 1 discloses a configuration in which a support body is inserted so as to come into contact with a light receiving surface and a non-light receiving surface of a heat receiving pipe in order to reinforce the heat receiving pipe and improve heat transfer between the light receiving surface and the non-light receiving surface.